Flp is a site-specific recombinase of the lambda integrase family. It binds to specific DNA sites, then cuts the DNA strands and religates them in recombinant configuration, as below: Although the reaction mechanism is very similar to that of Cre recombinase (1), the sequence homology between the two proteins is extremely limited. Flp (from yeast) is the only eukaryotic member of this family and is expected to differ from cre in several intersting aspects, including the use of domain swapping in assembling the active site. The recombination reaction requires no cofactors, and the energy of the phosphodiester bonds of the DNA backbone is stored in a covalent phospo-tyrosine intermediate. Data was collected on BioCARS Station 14-BM-D.